Chair back



FIG. 1 is a perspective view of a chair back showing our new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof.

The broken lines shown in the figures illustrate portions of the chair back that form no part of the claimed design. 

CLAIM The ornamental design for a chair back, as shown and described. 